The technology of the present disclosure relates to a belt member, a fixing device, and an image forming apparatus including the same, and more particularly, to a belt member used in a fixing device of an electromagnetic induction heating system, a fixing device, and an image forming apparatus including the same.
In an image forming apparatus, a toner image formed on an image carrying member such as a photosensitive drum is transferred to a recording medium, the recording medium carrying the toner image is conveyed toward a fixing device, and the fixing device applies heat and pressure, so that an unfixed toner image on the recording medium is fixed to the recording medium. A fixing device includes electromagnetic induction heating system comprising a fixing roller, a belt member disposed on an outer peripheral surface of the fixing roller, a pressing roller brought into press contacted with the belt member, wherein an induction heating unit disposed facing the belt member to heat the belt member, and a toner image is fixed to the recording medium while the recording medium passes through a fixing nip portion between the belt member and the pressing roller.
According to the fixing device of the electromagnetic induction heating system, an eddy current is generated by magnetic flux generated by the induction heating unit to a heating layer provided in the belt member, the heating layer generates heat by the Joule heat generated by the eddy current, and the belt member is heated to a predetermined fixing temperature. In this type of fixing device, since the heat capacity of the heating layer can be reduced, a warm-up time for starting to operate the device can be shortened, so that a compact sized fixing device as well as high heat conversion efficiency can be obtained.
As the fixing device of the electromagnetic induction heating system, there have been known a uniaxial fixing device in which only a fixing roller is disposed on the inner peripheral surface of the belt member, and a multiaxial (biaxial) fixing device in which a fixing roller, a heat roller and the like are disposed on the inner peripheral surface of the belt member.
In the multiaxial (biaxial) fixing device, the fixing roller is provided at the outer peripheral surface thereof with an elastic layer, and forms a fixing nip portion. The heat roller thermally converts the magnetic flux generated by the induction heating unit and having passed through the belt member, thereby heating the belt member. As described above, although the magnetic flux generated by the induction heating unit has passed through the belt member, since the magnetic flux is thermally converted by the heat roller and the belt member is heated, it is possible to reduce power loss.
On the other hand, in the uniaxial fixing device, if the magnetic flux generated by the induction heating unit passes through the belt member, a cored bar of the fixing roller generates heat. However, since an elastic layer is formed on the outer peripheral surface side of the fixing roller, the fixing roller is not able to heat the belt member. Therefore, there is a case in which power loss occurs, and the temperature of the cored bar of the fixing roller is excessively raised, and thus the elastic layer is degraded and is broken. In addition, the uniaxial fixing device has a merit that it is possible to limit the entire heat capacity as compared with the multiaxial (biaxial) fixing device.
As the belt member used in the aforementioned fixing device of the electromagnetic induction heating system, various belt members have been known.
For example, there has been known a belt member (a first conventional structure) provided with a base layer (a thickness: 40 μm to 50 μm) including a magnetic metal such as Ni and an elastic layer and a release layer sequentially stacked on the base layer. In the belt member, the thickness of the base layer including a magnetic metal is limited to 40 μm to 50 μm, so that the bending performance of the belt member necessary for forming a fixing nip portion is maintained. However, since it is not possible to sufficiently ensure the thickness of the base layer with respect to the skin depth of the magnetic metal, leakage magnetic flux (magnetic flux passing through the belt member) is slightly generated, resulting in the degradation of heating performance.
In addition, the skin depth is a depth at which magnetic flux is converted into an eddy current and is attenuated to 1/e (e is the base of natural logarithms), wherein a skin depth δ[m]=1/√(πfμσ). f denotes a frequency [Hz], μ denotes permeability [H/m], and σ denotes conductivity [s/m]. The eddy current mainly flows through a thickness part equal to or less than the skin depth. Accordingly, when the thickness is equal to or more than the skin depth, the eddy current mainly flows in a range of the skin depth or less, and when the thickness is equal to or less than the skin depth, the eddy current flows in the whole thickness direction. In the magnetic metal, since the permeability is large and the skin depth is small, a resistance value at an obtained skin depth reaches a level suitable for the generation of an eddy current loss, so that high heating performance is obtained at a thickness equal to or more than the skin depth. On the other hand, in a non-magnetic metal, since the permeability is small and the skin depth is large, a resistance value at an obtained skin depth is too low, so that high heating performance is not obtained, but when the thickness is made smaller than the skin depth, a resistance value is increased, and heating performance indicates a peak at a predetermined thickness.
For example, as shown in the following Table 1, when a use frequency f of the induction heating unit is 20 kHz to 50 kHz, the skin depth of Ni (relative permeability μr≈180 and conductivity σ=1.5E7[s/m]) is around 50 μm, and has a value approximate to the thickness used in the belt member including Ni. On the other hand, the skin depth of Cu (relative permeability μr≈1 and conductivity σ=5.8E7[s/m]) is about 300 μm or more, and when the depth is equal to or less than about 300 μm, an eddy current flows in the whole thickness direction. An effective resistance value at this time is calculated as a reference value. The effective resistance value of Ni is 9.7E-4 to 1.5E-3, and the effective resistance value of Cu is 3.7E-5 to 5.8E-5. In addition, the effective resistance value [Ω] is defined as resistivity ρ [Ω·m]/thickness [m]=1/(conductivity σ [s/m]×thickness [m]). The effective resistance value is an index proportional to a cross-sectional resistance value in a direction parallel to the thickness of the belt member.
TABLE 1Frequency [kHz]203050NoteNi skin depth [μm]685543Cu skin depth [μm]480380290Ni effective9.7E−41.2E−31.5E−3 Thickness = skinresistance value [Ω]depthCu effective3.7E−54.5E−55.8E−5Thickness = skinresistance value [Ω]depthCu effective1.7E−3Thickness = 10 μmresistance value [Ω]
In Table 1 above, the effective resistance values of Cu are calculated in two conditions in which the thickness is the skin depth and is 10 μm. When the effective resistance values at the skin depths are compared with each other, Cu is 1/10 or less with respect to Ni, and the thickness of Cu is thinned to about 10 μm, so that the effective resistance value of a Cu layer is increased to a level of the effective resistance value of the skin depth of a Ni layer. Referring to the effective resistance values of Table 1 above, it is considered that about 1 mΩ to about 2 mΩ are effective resistance value levels necessary for a heating member.
In the magnetic metal, the relation of the image diagram illustrated in FIG. 5 is established between the thickness and the heating performance, the leakage magnetic flux, and the effective resistance value. Accordingly, when the thickness is equal to or more the skin depth, since an eddy current mainly flows in a range of the skin depth, the heating performance is controlled at the skin depth.
On the other hand, in the non-magnetic metal, as in the image diagram illustrated in FIG. 6, the heating performance is easily dependent on the thickness. In the case of Cu, a heating peak is expressed by a thickness of about 5 μm to about 15 μm. Accordingly, the heating performance of the non-magnetic metal is controlled by the thickness in a range of the skin depth or less.
When a non-magnetic metal layer is used as an IH heating member, since it is used in a thickness equal to or less than a skin depth, leakage magnetic flux passing through a heating layer is generated to a certain degree. As a method for improving the heating performance and the leakage magnetic flux of the non-magnetic metal layer, a method for stacking the non-magnetic metal layer together with a magnetic metal layer has been known. For example, there has been known a belt member (a second convention structure) provided with a base layer (a thickness: 30 μm to 35 μm) including a magnetic metal such as Ni, a non-magnetic metal layer (a thickness: 5 μm to 15 μm) including Cu and the like and stacked on the base layer, and an anti-oxidation film, an elastic layer, and a release layer sequentially stacked on the non-magnetic metal layer. In this belt member, heat is generated by both the base layer including the magnetic metal and the non-magnetic metal layer. Since the base layer has a smaller thickness as compared with the base layer of the first convention structure, the heating performance is slightly degraded. However, since the non-magnetic metal layer has the same heating performance as that of the base layer of the first convention structure, it is possible to improve the heating performance in the second convention structure, as compared with the first convention structure.
In the case of a belt member using a metal base layer, it is preferable that the total thickness of a metal layer is about 50 μm or more in terms of heating performance. On the other hand, in terms of bending performance, it is preferable that the total thickness of the metal layer is about 50 μm or less. In the present circumstances, in order to balance these two performance, a belt member in which the total thickness of the metal layer is about 40 μm to about 50 μm is mainly used. However, there has also been a demand for further improving the bending performance of the belt member.
Therefore, for example, there has been proposed a belt member (a third convention structure) provided with a base layer (a thickness: 50 μm to 100 μm) including an insulating resin such as polyimide, a non-magnetic metal layer (a thickness: 5 μm to 15 μm) including Cu and the like and stacked on the base layer, and an anti-oxidation film, an elastic layer, and a release layer sequentially stacked on the non-magnetic metal layer. In this belt member, the base layer is made of an insulating resin and the thickness of the metal layer is limited, so that it is possible to improve the bending performance.
In addition, there has been proposed a fixing belt (a belt member) provided with a base layer including a metal, a heating layer stacked on the base layer and including a metal, and a surface release layer stacked on the heating layer. Furthermore, there has been proposed a fixing belt (a belt member) in which a plurality of non-magnetic metal layers sequentially stacked are included and the total thickness of the non-magnetic metal layers is 48 μm to 63 μm.